Method and a device for cleaning of crankcase gas

ABSTRACT

Upon cleaning of crankcase gas generated during operation of an internal combustion engine in its crankcase, a centrifugal separator is used, which includes a rotor arranged for rotation by means of a driving motor and arranged by its rotation suck crankcase gas from the crankcase through a conduit to the centrifugal separator. During operation of the combustion engine a parameter, e.g. a measurement of the load on the combustion engine is sensed, the magnitude of said parameter being related to the amount of crankcase gas generated per unit of time in the crankcase. Depending upon a sensed change of the sensed parameter the rotational speed of the rotor of the centrifugal separator is changed in a way such that the gas pressure in the crankcase is maintained at a predetermined value, or within a predetermined pressure interval, during the operation of the combustion engine.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporates byreference essential subject matter disclosed in InternationalApplication No. PCT/SE2003/001031 filed on Jun. 17, 2003 and SwedishPatent Application No. 0201933.9 filed on Jun. 20, 2002.

FIELD OF THE INVENTION

The present invention relates to a method and a device for cleaning ofcrankcase gas that is generated during operation of an internalcombustion engine in its crankcase. Internal combustion engines are usedfor different purposes, e.g. for propelling of transportation means onland, on sea or in the air, for performing mechanical work or instationary or mobile plants for production of electric current.

BACKGROUND OF THE INVENTION

Cleaning of crankcase gas requires a cleaning device, which caneffectively separate very small particles, solid and/or liquid,suspended in the crankcase gas. Different types of cleaning devices havebeen proposed, such as traditional filters, cyclones or different kindsof centrifugal separators having rotating members. Lately, centrifugalseparators of a relatively advanced kind have been proposed for suchcleaning, and different methods have been proposed for driving ofcentrifugal separators of these kinds. Thus, it has been suggested thata centrifugal separator for this purpose should be driven mechanicallyby means of one of the ordinary shafts of the combustion engine, e.g.the crank shaft or the cam shaft (see for instance U.S. Pat. No.5,954,035). Another suggestion has been that a centrifugal separatorshould be driven by means of an electric motor (see for instance WO01/36103). A further suggestion has been that a fluid, gas or liquid,should be pressurized by means of the combustion engine and be used fordriving of a turbine of one kind or another, coupled to a centrifugalrotor for cleaning of crankcase gas (see for instance WO 99/56883).

Irrespective of the kind of device used for cleaning of crankcase gasthat is produced by a combustion engine it is difficult to avoid thatthe operation of the cleaning device influences the crankcase gaspressure prevailing in the crankcase of the combustion engine. Eitherthe cleaning device creates a counter pressure for the crankcase gas tobe cleaned, which may lead to an undesired overpressure in thecrankcase, or the cleaning device causes a certain underpressure in itsinlet for crankcase gas to be cleaned, which may propagate to thecrankcase of the combustion engine and created therein an undesiredunderpressure. In many cases there is a requirement that the pressure inthe crankcase must be kept within a certain pressure interval, i.e. itmust not-rise above a certain first value and must not drop below acertain second value during the operation of the combustion engine.

The said problem may theoretically be solved for instance by means ofpressure sensing control valves of different kinds. However, greatdemands are put on the pressure sensitivity of such valves, and thesedemands may be difficult to fulfil in the environment where the valveshave to operate.

The object of the present invention is to provide a method and a devicefor cleaning of crankcase gas generated during operation of a combustionengine in its crankcase, by means of which a predetermined gas pressure,or a gas pressure within a predetermined pressure interval, ismaintained in the crankcase of the combustion engine.

SUMMARY OF THE INVENTION

The present invention is directed in one aspect to the use of acentrifugal separator that includes a centrifugal rotor arranged forrotation by means of a driving motor and arranged by its rotation tosuck crankcase gas from the crankcase to the centrifugal separator,

-   -   sensing of a parameter, the magnitude of which is related to the        amount of crankcase gas generated per unit of time in the        crankcase, and    -   changing the rotational speed of the centrifugal rotor in        response to a sensed change of said parameter in a way so that        the gas pressure in the crankcase is maintained at a        predetermined value, or within a predetermined pressure        interval, during the operation of the combustion engine. The        change of the rotational speed of the centrifugal rotor may be        accomplished stepwise or continuously.

By the invention it is possible to accomplish satisfactory cleaning ofcrankcase gas from the combustion engine even when the load thereonvaries, while a desired gas pressure is maintained in the crankcase ofthe combustion engine. The invention is based on the understanding thata centrifugal separator that is used need not be operated with the sameseparation efficiency during the complete operating time of thecombustion engine and that the cleaning efficiency and the suctionefficiency of the centrifugal separator can be controlled by changingthe rotational speed of the centrifugal rotor. Thus, when a relativelysmall amount of crankcase gas is generated per unit of time, efficientcleaning of the crankcase gas may be accomplished at a substantiallylower rotational speed of the centrifugal rotor than is needed when arelatively large amount of crankcase gas is generated per unit of time.

By use of a centrifugal separator, the centrifugal rotor of which isarranged upon its rotation to suck crankcase gas from the crankcase, theneed of an extra pump or fan for transportation of crankcase gas fromthe crankcase to and through the centrifugal separator is avoided.

Upon use of a combustion engine for propelling a vehicle a givenrelation often prevails between the speed, by which the vehicle ispropelled, and the amount of crankcase gas being generated in thecrankcase of the combustion engine. This need not always be the case,however, and it is usually not the case when a combustion engine is usedfor instance for production of electric current. Thus, the morecrankcase gas generated in the crankcase the more load there is put onthe combustion engine, independent of whether the combustion enginecrank shaft is driven at an increased or at a substantially unchangedrotational speed. In connection with production of electric current,i.e. when the combustion engine is arranged for rotation of anelectrical generator, the combustion engine may be kept in operationsubstantially at an unchanged rotational speed upon varying productionof electric current. In this case the electrical generator is adjustableduring operation in order to be able to generate, at a constantrotational speed of the combustion engine, different amounts of electriccurrent according to a varying need of such current.

Sensing of a change of the amount of crankcase gas generated per unit oftime in the crankcase of the combustion engine may be performed indifferent ways. For instance, the gas pressure in the crankcase or in aconnection between this and the centrifugal separator may be sensed.Alternatively, the flow of crankcase gas in said connection can besensed.

Alternatively, the said sensed parameter may be of a kind indicating theprevailing or an immediately approaching load on the combustion engine.In connection with production of electric current by means of anelectric generator operated by the combustion engine, thus, afterpreparative empirical trials, control of the centrifugal rotor speed canbe accomplished on the basis of continuous sensing of a varying need ofelectric current, that is satisfied by the electrical generator. On thebasis of knowledge about how such a varying need influences the load onthe combustion engine, and thereby the amount of crankcase gas producedper unit of time, the rotational speed of the centrifugal rotor may beadjusted in a suitable way. For instance, the effect delivered by theelectrical generator may be sensed and caused to generate a controlsignal that varies with changes of this effect. The magnitude of thecontrol signal can be regarded as a measurement of the load on thecombustion engine driving the electrical generator and, thereby,representative for the amount of crankcase gas generated in thecrankcase of the combustion engine. In a practical case a control signalmay vary within the interval of 4-2O mA depending upon the load of theelectrical generator on the engine, and the corresponding speed intervalfor the rotation of the centrifugal rotor may be such that the gaspressure in the crankcase of the combustion engine is kept within thepressure interval 0-4 mbar.

As mentioned initially a centrifugal rotor for cleaning of crankcase gasmay be driven in many different way. The invention is applicableindependent of which kind of device is chosen for driving of thecentrifugal rotor. Advantageously, an electrical motor is used for thedriving of the centrifugal rotor, and in a case like this a frequencyconverter is preferably used for changing of the rotational speed of themotor.

On a modern vehicle of the category large lorries and working machinesthere is normally a computer network installed. One previously knowncomputer network of this kind is called CAN-bus (CAN=Controller AreaNetwork). In this computer network, which preferably includes a vehiclecomputer and is coupled to many different sensors placed in differentparts of the vehicle and in the combustion engine, there are a lot ofdata available concerning various functions and conditions prevailing onthe vehicle. Examples of data, which may be available are:

-   -   the present engine speed    -   the present load on the engine    -   the load on the engine as desired by the driver of the vehicle        (present pressure on the accelerator pedal of the vehicle)    -   the engine temperature    -   the ambient air temperature    -   the speed of the vehicle    -   the total operational time of the engine since the engine was        new    -   the driving distance of the vehicle since the engine was new    -   operational time since the engine was last started

Of course, many other data are available in a network of this kinddepending upon which components are present on the vehicle. By means ofa vehicle computer connected with the network various data may becombined and converted into control signals of various kinds forcontrolling of various functions on board the vehicle, e.g. signals forcontrolling of the cleaning of crankcase gas according to the invention.

In a preferred embodiment of a device according to the invention itsequipment for changing the rotational speed of the centrifugal separatormay comprise a decoding- or selection device, that is adapted to receiveor choose only certain ones of several data available in a source ofinformation of this kind, said control equipment being adapted to beactuated by such received or chosen data and, in turn, to change therotational speed of the electrical motor in dependence thereof.Irrespective of whether a computer network of the aforementioned kind isavailable or not the cleaning device in an advantageous embodiment ofthe invention is preferably connectable to a sensor that is arranged tosense an actual change of the amount of crankcase gas being produced bythe combustion engine.

The invention is described more closely in the following with referenceto the accompanying drawing, in which FIG. 1 shows an example of anelectrical motor driven centrifugal separator of the kind that can beincluded in a device according to the invention, FIG. 2 is a sectionalview taken along the line II-II in FIG. 1, FIG. 3 illustratesschematically a vehicle equipped with a device according to theinvention and FIG. 4 shows schematically a plant for production ofelectric current, including among other things an internal combustionengine and a device according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of a centrifugal separator that may bemounted on a vehicle and is intended for cleaning of crankcase gas fromparticles suspended therein, which have a larger density than the gas.

FIG. 2 illustrates a separation disc forming part of the centrifuge ofFIG. 1.

FIG. 3 schematically illustrates a vehicle incorporating the centrifugalseparator of FIG. 1.

FIG. 4 schematically illustrates a plant for the production of electricequipment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The centrifugal separator includes a housing 1 delimiting a chamber 2.The housing forms a gas inlet 3 to the chamber 2 for gas to be cleanedand a gas outlet 4 from the chamber 2 for clean gas. The housing furtherforms a particle outlet 5 from the chamber 2 for particles having beenseparated from the gas.

The housing 1 includes two parts which are kept together by means of anumber of screws 6. These screws 6 are also adapted to fix the housingto suspension members 7 of some elastic material, through which thehousing may be supported on said vehicle (not shown).

Within the chamber 2 a rotor 8 is arranged rotatable around a verticalrotational axis R. An electrical motor 9 is arranged for rotation of therotor 8. The rotor 8 includes a vertically extending central spindle 10,which at its upper end is journalled in the housing 1 through a bearing11 and a bearing carrier 12 and at its lower end is journalled in thehousing I through a bearing 13 and a bearing carrier 14. The bearingcarrier 14 is situated in the gas inlet 3 of the housing and istherefore provided with through holes 15 for incoming gas to be cleanedin the chamber 2.

The rotor 8 further includes an upper end wall 16 and a lower end wall17, said two end walls being connected with the central spindle 10. Thelower end wall 17 in a central portion is provided with through holes18, so that the interior of the rotor may communicate with the gas inlet3. Furthermore, the lower end wall 17 is provided with an annular flange19 that is arranged to cooperate with a similar annular flange 20 of thebearing carrier 14, so that gas entering through the gas inlet 3 isconducted into the interior of the rotor 8 through the just mentionedholes 18. The flanges 19 and 20 may be arranged to seal completelyagainst each other, but a complete sealing between them is notnecessary.

The lower end wall 17 is shaped in one piece with a hollow column 21,which extends axially upwardly from the end wall 17 and closelysurrounds the central spindle 10. The column extends all the way up tothe upper end wall 16.

In the area of the column 21 the central spindle 10 is cylindrical,preferably for cost reason circular cylindrical, and the inside of thecolumn 21 is formed in the same way as the outside of the spindle. Theoutside of the column 21 has a non-circular cross sectional shape, ascan be seen from FIG. 2.

Between the end walls 16 and 17 there is arranged a stack of conicalseparation discs 22. Each one of these has one portion formed as afrustum of a cone and one plain portion 23 formed integrally therewith,closest to the column 21. As shown in FIG. 2, the plain portion isformed so that it may engage with the non-circular column 21 in a waysuch that the separation disc shall not be able to rotate relative tothe column 21. Furthermore, the plain portion 23 is provided withseveral through holes 24. Independent of whether the holes 24 in thevarious separation discs 22 are axially aligned or not they formtogether with the interspaces between the central portions of theseparation discs 22 a central inlet space 25 within the rotor (see FIG.1), which communicates with the gas inlet 3.

For the sake of clarity the drawing shows only a few separations discs22 having large axial interspaces. In practice, many more separationdiscs are to be arranged between the end walls 16 and 17, so thatrelatively thin interspaces are formed there between.

FIG. 2 shows the side of a separation disc 22, that is facing upwardlyin FIG. 1. This side is called the inside of the separation disc in thefollowing, since it is facing in a direction inwardly towards therotational axis of the rotor. As can be seen, the separation disc on itsinside is provided with several elongated ribs 26 forming spacingmembers between this separation disc and the separation disc situatedclosest above in the disc stank. Between adjacent ribs 26 in aninterspace between two separation discs there are formed flow passages27 for gas to be cleaned. The ribs 26 extend, as shown in FIG. 2, alongcurved paths and form at least at the radially outer circumferencialportions of the discs an angle with the generatrices of the separationdiscs. As a consequence of the curved form of the ribs 26 also the flowpassages 27 for the gas to be cleaned extend along paths which arecurved in a corresponding way. The ribs 26 preferably extend acrosssubstantially the whole conical portion of each separation disc and endup in the vicinity of the radially outer surrounding edge of theseparation disc.

An annular space 28 surrounds the rotor 8 in the housing 1 andconstitutes part of the chamber 2.

The centrifugal separator described above is previously known and itsfunction is extensively described in WO 01/36103. This function can bedescribed briefly, as follows.

By means of the electrical motor 9 the rotor 8 is brought into rotationaround the vertical axis R. Crankcase gas that has been produced by acombustion engine (not shown) and that is to be freed from oil particlesand possibly also solid particles suspended therein, enters through thegas inlet 3 and is conducted upwardly in the central space 25 in therotor 8. From there the crankcase gas is conducted into the interspacesbetween the conical portions of the separation discs 22, where it isentrained in the rotor rotation. As a consequence of the rotation theparticles suspended in the gas are separated by being thrown by thecentrifugal force against the insides of the separation discs, on whichthey then slide, or run in the form of a liquid, radially outwardlytowards the surrounding edges of the separation discs. From thesesurrounding edges the particles in an agglomerated or coalesced form arethrown outwardly towards the surrounding wall of the stationary housing1, along which they move downwardly and further out through the particleoutlet 5.

The crankcase gas having been freed from particles flows out from theinterspaces between the separation discs 22 to the annular space 28,which its leaves through the gas outlet 4.

The separation or cleaning efficiency of the centrifugal separator to alarge extent depends on the rotational speed of the centrifugal rotor 8.The larger this speed is, the larger becomes the separation efficiency.By controlling of the rotational speed of the electrical motor 9 thecleaning efficiency of the centrifugal separator may be changed. Even ina non-rotating state the centrifugal rotor has a certain, althoughsmall, cleaning effect as a consequence of the crankcase gas beingforced to change its direction of flow several times and to flow throughnarrow channels upon its passage through the centrifugal rotor.

By its construction the described centrifugal separator is adapted tosuck impure crankcase gas in through its gas inlet 3 and to pump, by acertain overpressure, cleaned crankcase gas out through its gas outlet4. The reason for this is that entering crankcase gas, that is notrotating, is conducted into the central inlet space 25 of thecentrifugal rotor and is thereafter brought into rotation while it iscleaned on its way between the separation discs 22 out towards theannular space 28 around the rotor B in the housing 1. Crankcase gas inthe surrounding space 28, thus, has a higher pressure than crankcase gasin the central space 25. It is important in this connection that the gasoutlet 4 is situated at a larger distance from the rotational axis R ofthe centrifugal rotor than is the central gas inlet 3.

FIG. 3 illustrates a vehicle 30 and an internal combustion engine 31supported thereby and arranged for the propelling of the vehicle. Thecombustion engine 31 is arranged also for operation of a currentgenerator 32 that is connected with a current accumulator 33. Thevehicle 30 is also equipped with a centrifugal separator 34 of the kindshown in FIGS. 1 and 2. A conduit 35 is arranged for conductingcontaminated crankcase gas from the crankcase of the engine 31 to thegas inlet of the centrifugal separator, and conduits 36 and 37 arearranged for conducting cleaned crankcase gas and particles and oilseparated from the crankcase gas, respectively, back to the combustionengine. The cleaned gas is conducted to the air intake of the combustionengine, and the separated particles are conducted together with theseparated oil back to the so-called oil tray of the combustion engine.Alternatively, the cleaned gas could have been released to thesurrounding atmosphere, while the separated particles and oil could havebeen collected in a separate vessel.

FIG. 3 also shows the electrical motor 9, which can be seen from FIG. 1and which is arranged for driving the rotor 8 of the centrifugalseparator. In connection to the electrical motor 9 there is arranged acontrol equipment 38 that is adapted to drive the electrical motor 9 atvarying speeds. The control equipment 38 for current supply to itselfand to the electrical motor is connected to the current generator 32 andto the current accumulator 33.

Further installed on the vehicle 30 is a computer network including avehicle computer 39 and a so-called data-bus 40. A large number ofsensors of various kinds are connected to the computer network forcollection of data concerning different functions on the vehicle. Alsosaid control equipment 38 is connected to this computer network, fromwhich information can be collected for connection and disconnection ofthe electrical motor 9 or for change of the rotational speed of theelectrical motor by means of the control equipment. Through theconnection to the computer network information can also be given theretoof, for instance, the condition and operational status of the electricalmotor and the centrifugal separator and the degree of contaminants incontaminated and cleaned crankcase gas.

The aforementioned motor 9 may be a direct-current motor or analternating-current motor; either a synchronous motor or an asynchronousmotor. Depending upon the type of the electrical motor the controlequipment 38 may be designed in many different ways self-evident for aperson skilled in the art of electrical motors.

If the control equipment 38 is connected to a computer network of theabove mentioned kind, it does not have to be particularly complicatedwith a regard to its ability to treat signals coming from differentsensors on the vehicle. In a case like that, namely, the aforementionedvehicle computer is equipped for a required such signal treatment andfor production of a control signal for the driving of the electricalmotor. The control equipment 38 in this case has to include a decodingmeans (interface), by means of which it can choose a correct signal fromthe computer network, after which the signal can be used for thecontrolling of the rotational speed of the electrical motor.

In the simplest case the control equipment may include an electricalrelay, which is arranged for starting or stopping the operation of theelectrical motor by means of a received control signal. A relay of thiskind may have an input circuit influencable by data intended forcontrolling of the separation efficiency of the centrifugal separator34, and an output circuit arranged for changing of the rotational speedof the electrical motor 9 depending upon these data.

However, the control equipment preferably includes a device for drivingthe electrical motor 9 at different speeds; either so that a limitednumber of speeds can be obtained or so that a continuous change of themotor speed can be performed. Different kinds of devices for speedregulation of motors (both direct-current and alternate-current motors)are well known and need no closer description here. For a direct-currentmotor a simple device for voltage control may be used. For analternate-current motor various kinds of frequency control equipment maybe used. Equipment of this kind is available for production of alternatecurrent of variable frequency either by means of direct current or bymeans of alternate current.

Irrespective of whether the control equipment is of a sophisticated kindor not, it should be of a kind such that the operation of the electricalmotor can be interrupted while the combustion engine is still inoperation, e.g. at idle driving or upon operation of the combustionengine at a speed lower than a certain value. If desired, the operationof the electrical motor may be manually disconnectable and/orautomatically connectable a certain period of time after the combustionengine has been started or after a certain rotational speed of thecombustion engine has been reached. The electrical motor should beadapted for operation at a voltage of 48 volts or lower, e.g. 14, 28 or42 volts.

As to the signal, by means of which the speed of the electrical motorshould be controlled or adjusted, it may be a function of many differentvariable factors. Thus, one or more of the following factors may beincluded, for instance:

-   -   the gas pressure in the crankcase of the combustion engine    -   the gas pressure in the air intake of the combustion engine    -   the rotational speed of the combustion engine    -   the load on the combustion engine    -   the ambient air temperature    -   the lubricating oil temperature of the combustion engine    -   the total operational time of the combustion engine

If there is no computer network of the kind described above, therotational speed of the electrical motor may be adjusted or controlledby means of data transferred to the control equipment 38 directly from asensor of some suitable kind. For instance, a sensor may be arranged inone way or another to sense the amount of crankcase gas being producedeach moment by the combustion engine. A measurement of the amount ofcrankcase gas produced may be constituted by the pressure prevailing inthe crankcase of the combustion engine or in a passage for crankcase gasbetween the crankcase and said centrifugal separator. Anothermeasurement of the production of crankcase gas may be constituted by avalue of the prevailing gas flow in said passage.

If the amount of produced crankcase gas is changed without therotational speed of the centrifugal rotor 8 being changed, the pressureof the crankcase gas in the gas inlet 3 of the centrifugal separator ischanged. Thus, if the production of crankcase gas decreases, the rotor 8at a maintained rotational speed will more effectively than before suckcrankcase gas from the combustion engine, whereby the pressure in thegas inlet 3 will drop. This means that also the gas pressure in thecrankcase of the combustion engine 31 will drop. A pressure drop of thiskind is avoided, however, by the previously described control equipment38 causing the electrical motor 9 to reduce its rotational speed andthereby the rotational speed of the centrifugal rotor 8.

Even at a rotational speed reduced in this way the centrifugal rotor 8will effectively clean the crankcase gas flowing therethrough. This ispossible because at this stage a reduced amount of impure crankcase gasis produced by the combustion engine 31 per unit of time, and thiscrankcase gas therefore will be given an increased maintenance time inthe centrifugal rotor on its way between the separation discs 22.Hereby, it will be given time to be effectively freed from oil and sotparticles in spite of the centrifugal rotor 8 rotating at a reducedspeed.

When the control equipment 38 gets a signal that the production ofcrankcase gas increases again, the rotational speed of the centrifugalrotor 8 will increase.

FIG. 4 shows schematically a plant for production of electric current.The plant may be stationary and land based or be mobile and be arrangedfor instance on board a vessel. The plant includes an internalcombustion engine 41 arranged to drive through a transmission 42 anelectrical generator 43. The electrical generator is arranged to delivercurrent through lines 44, an effect measuring device 45 and connections46 to a current consumption plant (not shown).

The rotational speed of the combustion engine 41 may be varied but ispreferably maintained substantially constant during operation of theplant. The electrical generator 43 is of a kind that may supply avarying effect at a substantially constant speed of the combustionengine. A variation of the effect output from the electrical generator43 leads to a corresponding variation of the load by the electricalgenerator on the combustion engine 41, the rotational speed of which ismaintained unchanged in spite of this.

When there is an effect output from the electrical generator 43, whichis sensed by means of the effect measuring device 45, there is created aweak control current conducted through connections 47 to a frequencyconverter 48. The magnitude of the control current varies with themagnitude of the effect output, i.e. with the magnitude of the load onthe combustion engine 41. The frequency converter is connected to anelectrical motor 49 arranged to drive the centrifugal rotor of acentrifugal separator 50 at varying speed. The centrifugal separator 50is of the same kind as the centrifugal separator shown in FIGS. 1 and 3.

Crankcase gas is conducted from the crankcase of the combustion engine41 through a conduit 51 into the centrifugal separator 50, thecentrifugal rotor 8 sucking crankcase gas to and in through its gasinlet (see FIG. 1). Cleaned crankcase gas is returned through a conduit52 from the gas outlet 4 of the centrifugal separator to the combustionengine 41. Oil and some solid particles separated from the crankcase gasare returned through a conduit 53 from the particle outlet 5 of thecentrifugal separator to the combustion engine 41.

When the combustion engine 41 is subjected to a relatively heavy load bythe electrical generator 42, a relatively large amount of crankcase gasis generated in its crankcase per unit of time. The crankcase gas iscleaned in the centrifugal separator 50, the centrifugal rotor of whichat this stage is kept in rotation by the motor 49 at a relatively highrotational speed. This rotational speed, that is determined by a certainvalue of the control signal in the connections 47 and of the frequencycontrol signal from the frequency converter 48 to the motor 49,generated by the control current, is sufficiently high for thecentrifugal separator to be able to receive and clean the whole amountof crankcase gas thus generated, while a gas pressure of an acceptablemagnitude is maintained in the crankcase of the combustion engine.

At a different stage, when the combustion engine is subjected to arelatively small load by the electrical generator 43, a relatively smallamount of crankcase gas is generated per unit of time in the crankcaseof the combustion engine. Then there is created in the connections 47 acontrol signal of a different value than at the higher load on thecombustion engine, and the frequency converter 48 is caused to reduce,in a corresponding degree, the rotational speed of the rotor of thecentrifugal separator 50. Hereby, the suction effect of the centrifugalrotor in the crankcase gas conduit 51 is adapted to the amount ofcrankcase gas generated at this new stage in the crankcase, whereby isavoided that a heavy underpressure is created in the crankcase, so thata gas pressure within a desired pressure interval is still maintained inthe crankcase.

1-11. (canceled)
 12. A method of cleaning crankcase gas generated duringoperation of an internal combustion engine in its crankcase, said methodincluding the steps of: using centrifugal separator that includes acentrifugal rotor arranged for rotation by means of a driving motor andarranged by its rotation to suck crankcase gas from the crankcase to thecentrifugal separator, sensing a parameter, the magnitude of which isrelated to the amount of crankcase gas generated per unit of time in thecrankcase, and changing the rotational speed of the centrifugal rotor inresponse to a sensed change of said parameter such that gas pressure inthe crankcase is maintained at at least one of a predetermined value;and within a predetermined pressure interval, during the operation ofthe combustion engine.
 13. A method according to claim 12, in which therotational speed of the centrifugal rotor is changed from a first valueto a second value, both larger than zero.
 14. A method according toclaim 12, in which the rotational speed of the centrifugal rotor ischanged stepwise in more than two steps.
 15. A method according to claim12, in which the rotational speed of the centrifugal rotor is changedcontinuously.
 16. A method according to claim 12, in which therotational speed of the centrifugal rotor is changed in response to asensed change of a flow of crankcase gas, that is generated as aconsequence of the production of crankcase gas by the combustion engine.17. A method according to claim 12, in which the rotational speed of thecentrifugal rotor is changed in response to a sensed change of an overpressure of the crankcase gas, which comes up as a consequence of theproduction of crankcase gas by the combustion engine.
 18. A methodaccording to claim 12, in which a parameter varying with the load on thecombustion engine is sensed.
 19. A method according to claim 12, inwhich an electric driving motor is used for the rotation of thecentrifugal rotor and a frequency converter is used for changing therotational speed of the driving motor and, thereby, the speed of thecentrifugal rotor.
 20. A device for cleaning crankcase gas generatedduring operation of an internal combustion engine, said deviceincluding; a centrifugal separator having a centrifugal rotor that isarranged for rotation by means of a driving motor and arranged by itsrotation to suck crankcase gas from the crankcase to the centrifugalseparator, the driving motor being arranged for rotation of thecentrifugal rotor at different speeds, sensing means for sensing aparameter, the magnitude of which is related to the amount of crankcasegas generated per unit of time in the crankcase, said sensing meansbeing operatively connected with the driving motor; and the drivingmotor being arranged to change the rotational speed of the centrifugalrotor in response to a sensed change of said parameter in a way suchthat the gas pressure in the crankcase is maintained at at least one ofa predetermined value, and within a predetermined pressure interval,during the operation of the combustion engine.
 21. A device according toclaim 20, in which the driving motor is electric and a frequencyconverter is connected between the sensing means and the driving motor.22. A device according to claim 20, in which the sensing means isarranged to sense a parameter that is related to the load to which thecombustion engine is subjected during its operation.